In a mass spectrometer, a collision cell can be used for a variety of purposes. For instance, a collision cell can be used to reduce the thermal energy of ions, to permit more accurate mass analysis thereby.
Collision cells can also be used in tandem mass spectrometry. In such techniques, structural elucidation of ionised molecules is performed by using a mass spectrum produced in a first mass analysis step, then selecting a desired precursor ion or ions from the mass spectrum, ejecting the chosen precursor ions (or ion) to a collision cell where they are fragmented, and transporting the ions, including the fragmented ions, to a mass analyser for a second mass analysis step in which a mass spectrum of the fragment ions is collected. The method can be extended to provide one or more further stages of fragmentation (i.e. fragmentation of fragment ions and so on). This is typically referred to as MSn, with n denoting the number of generations of ions. Thus MS2 corresponds to tandem mass spectrometry.
An instrument that is suitable for a wide array of mass spectrometry and MSn experiments is described in WO-A-2006/103412. This instrument has a longitudinal axis, along which is located an ion source and a reaction cell. Ions generated by the source travel along the axis in a forwards direction and enter the reaction cell, where they are fragmented. The fragmented ions are then ejected from the collision cell in a backwards direction along the longitudinal axis. They can then be received in an intermediate ion trap, from where they can be ejected to an off-axis mass analyser. Such an arrangement, together with a reagent ion source can be used for Electron Transfer Dissociation (ETD). A similar, but slightly different design of mass spectrometer is shown in U.S. Pat. No. 7,297,939.
Collision cells typically comprise electrodes for trapping ions and are pressurised and filled with gas to cause collisions. As a result, even if only fragmentation of ions is desired, collisional damping of the ion motion will nevertheless occur, such that the temperature of the ions is significantly reduced. Ejection of the ions in a backwards direction is therefore problematic. As explained in WO-A-2006/103412, ejection of the fragmented ions from the collision cell back along the longitudinal axis can be achieved by applying an accelerating DC potential gradient across the end-electrodes of the collision cell.
An alternative arrangement is described in GB-2389704, in which a collision cell comprises a plurality of ring-shaped electrodes. Ions are ejected by providing a DC axial gradient to these electrodes, preferably in a stepped way between the electrodes.
However, it has been found for existing arrangements that provide an axial gradient that the rate at which ions are ejected from the collision cell once trapped, or in the reverse direction, is much lower than would be expected.